The present invention relates to controlled-release and delayed delivery compositions. More particularly, the present invention relates to controlled-release compositions comprising acids.
It is well-known that concentrated acids present unique problems both in handling and in use. Acids such as concentrated sulfuric, phosphoric, hydrobromic, and hydrochloric are extreme safety hazards, potentially causing serious or even fatal injury if they are spilled onto skin or clothing. These acids, in concentrated form, are very irritating to tissue. In use great care must be employed when adding the acids to water solutions, for example, because of the explosive acid splatter that results due to rapid heat evolution. Shipping and storage also present problems. The acids are highly corrosive to metals and must generally be protected in glass-lined equipment, which is not always practical. For example, use in oil wells for generation of carbon dioxide from carbonate formations in many instances necessitates pumping the acid through more than a mile of steel pipe. At present corrosion inhibitors are used to slow down the attack of the acid on the steel. However, this is generally only partially successful and is limited to applications at mild temperatures.
Thus, it would be desirable to have the acid safely locked up in encapsulated, e.g., coated, particles of some type in a water slurry during the pumping process in the oil well, and then to be able to effect or predict complete release of the acid payload after arrival in the underground oil-bearing formation.
Because of the dangers involved in handling of concentrated acids in particular, and the corrosion problems that may be encountered in their transport and use, it has been found to be desirable to develop a composition and method of making these acids more easily and safely handleable. The present invention is such a composition and method, and is an encapsulated form of the liquid acid which can be released by a variety of mechanisms.
Encapsulation in general involves the formation of a protective wall of some type around a small particle, agglomeration of particles, or droplet of agent material. The wall is composed of a material suitable to achieve this goal, the material varying according to the degree of permeability and elasticity needed, the type of undesirable reactions to be avoided, and a number of other variables, each of which must be considered to ensure the best wall material choice. It is known in the art that, to encapsulate a liquid, it is generally necessary to first freeze and/or dry it, use a thickening additive, or absorb it into a porous matrix such as a porous clay. Then the wall material can be applied by one of a variety of processes, also depending in part on the agent material selected.
One method commonly employed for encapsulation of various agent materials involves the use of fluidized beds. In this process particles of the material to be encapsulated are sprayed with wall material while they are suspended in an air stream. The wall material sprayed can be of a polymer solution, molten wax, emulsion, suspension such as a latex, or other material, and is continued until the desired wall thickness is obtained. A design modification called the Wurster column can be employed to reduce particle agglomeration.
However, concentrated acids present fewer processing options because of their interaction with many potential wall materials. For example, water-based suspensions, when sprayed on concentrated acid droplets or absorbate particles, allow reaction between the acid and the water. This prevents formation of a proper coating and, in addition, the viscosity of the wall material in many cases tends to encourage agglomeration of the droplets or adsorbates, thereby inhibiting fluidization.
Regardless of the method or materials chosen, the goal is to produce a composition capable of controlled agent release, such as release at a predetermined time. This release can be immediate upon introduction into a selected environment, it can be delayed for a specific amount of time, or in the case of a number of agent particles or drops having differing wall thicknesses or compositions, it may be continuous over a period of varying length. The release is effected by varying mechanisms acting on the wall, such as by dissolution by the environment, reaction with the environment, or diffusion causing rupture of the capsule wall. U.S. Pat. No. 3,952,741, for example, illustrates a controlled delivery system based on osmotic bursting of a water-permeable wall
These mechanisms are effective for a wide variety of uses. For example, the rupture diffusion mechanism is particularly well-suited to applications where release of the agent is to be delayed for a predetermined amount of time and then effected fairly rapidly. This is particularly useful where local high concentrations are undesirable, and a uniformly-timed release is desired. Conversely, the dissolution method may be preferred for slower, somewhat sequential release where local high concentrations do not present problems However, regardless of the release mechanism chosen, the timing of release can be most precisely determined if the wall thicknesses over a sampling of a number of encapsulated particles are very uniform in thickness.